Photoelectric conversion element, imaging device, optical sensor and method of manufacturing photoelectric conversion element

ABSTRACT

To provide an organic photoelectric conversion element, imaging device, and optical sensor having low dark currents, and a method of manufacturing a photoelectric conversion element. Provided is a photoelectric conversion element, including: a first electrode; an organic photoelectric conversion layer disposed in a layer upper than the first electrode, the organic photoelectric conversion layer including one or two or more organic semiconductor materials; a buffer layer disposed in a layer upper than the organic photoelectric conversion layer, the buffer layer including an amorphous inorganic material and having an energy level of 7.7 to 8.0 eV and a difference in a HOMO energy level from the organic photoelectric conversion layer of 2 eV or more; and a second electrode disposed in a layer upper than the buffer layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/325,144, filed Jan. 10, 2017, now U.S. Pat. No. 10,109,813, which isa national stage application under 35 U.S.C. 371 and claims the benefitof PCT Application No. PCT/JP2015/062044 having an international filingdate of Apr. 21, 2015, which designated the United States, which PCTapplication claimed the benefit of Japanese Patent Application No.2014-147147 filed Jul. 17, 2014, the disclosures of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present technology relates to a photoelectric conversion element, animaging device and optical sensor including this photoelectricconversion element, and a method of manufacturing a photoelectricconversion element. More particularly, the present technology relates toan organic photoelectric conversion element, imaging device, and opticalsensor including an organic photoelectric conversion material, and amethod of manufacturing an organic photoelectric conversion element.

BACKGROUND ART

Conventionally, as an imaging element (image sensor), there is mainlyused a semiconductor element having a structure of a charge coupleddevice (CCD) or a complementary metal oxide semiconductor (CMOS). Also,there is recently proposed an imaging element including an organicphotoelectric conversion element having a photoelectric conversion layerformed with an organic semiconductor material (for example, see PatentLiteratures 1 to 3). The organic photoelectric conversion element doesnot need to contain a color filter, and can have a structure and amanufacturing process which are simpler than those of the conventionalinorganic semiconductor element.

The conventional organic photoelectric conversion element as describedin Patent Literatures 1 to 3 has a configuration in which an organicphotoelectric conversion portion is disposed between a pair ofelectrodes. For example, a lower electrode, an organic photoelectricconversion portion, and an upper electrode are laminated in this orderon a substrate. Also, the conventional organic photoelectric conversionelement includes various intermediate layers such as an electronblocking layer, a buffer layer and an active layer, between the organicphotoelectric conversion portion and each electrode in some cases.

CITATION LIST Patent Literature

Patent Literature 1: JP 2013-219190A

Patent Literature 2: JP 2014-063999A

Patent Literature 3: JP 2014-072328A

SUMMARY OF INVENTION Technical Problem

A photoelectric conversion element is required to have high conversionefficiency and low dark currents. However, the previously-describedconventional organic photoelectric conversion element has a problem thatsuppression of dark currents is not sufficient.

Therefore, a main object of the present disclosure is to provide anorganic photoelectric conversion element, imaging device, and opticalsensor having low dark currents, and a method of manufacturing aphotoelectric conversion element.

Solution to Problem

A photoelectric conversion element according to the present disclosureincludes: a first electrode; an organic photoelectric conversion layerdisposed in a layer upper than the first electrode, the organicphotoelectric conversion layer including one or two or more organicsemiconductor materials; a buffer layer disposed in a layer upper thanthe organic photoelectric conversion layer, the buffer layer includingan amorphous inorganic material and having an energy level of 7.7 to 8.0eV and a difference in a HOMO energy level from the organicphotoelectric conversion layer of 2 eV or more; and a second electrodedisposed in a layer upper than the buffer layer. The buffer layer may beformed with two or more metal oxides, for example. In that case, atleast one of the metal oxides may be a metal oxide semiconductor.Further, the buffer layer may be formed with two or more metal oxidesselected from the group consisting of zinc oxide, silicon oxide, tinoxide, niobium oxide, titanium oxide, molybdenum oxide, aluminum oxide,In—Ga—Zn-based oxides (IGZO), magnesium oxide and hafnium oxide. Thebuffer layer may be constituted by a plurality of layers each having adifferent energy level. The buffer layer may have a thickness of 3 to300 nm, for example. The buffer layer may have a surface resistance of100 kΩ/□ or more. The second electrode may be formed with a transparentmaterial. In that case, the buffer layer may also be formed with atransparent material, and may have a relative refractive index to thesecond electrode of 0.3 or less. The buffer layer may be formed by asputtering method, for example.

An imaging device according to the present disclosure includes theabove-mentioned photoelectric conversion element. Further, an opticalsensor according to the present disclosure includes the above-mentionedphotoelectric conversion element, and may be, for example, an infraredsensor.

A method of manufacturing a photoelectric conversion element accordingto the present disclosure includes a step of forming a first electrode;a step of forming, in a layer upper than the first electrode, an organicphotoelectric conversion layer including one or two or more organicsemiconductor materials; a step of forming, in a layer upper than theorganic photoelectric conversion layer, a buffer layer including anamorphous inorganic material and having an energy level of 7.7 to 8.0 eVand a difference in a HOMO energy level from the organic photoelectricconversion layer of 2 eV or more; and a step of forming, in a layerupper than the buffer layer, a second electrode. A sputtering method maybe applied to the step of forming a buffer layer. In that case, a filmof the buffer layer may be formed while introducing oxygen.

Advantageous Effects of Invention

According to the present disclosure, a high energy barrier can be formedto enhance a suppression effect of dark currents. It is noted that theeffects described herein are not necessarily limiting, and any one ofthe effects described in the present disclosure may be exerted.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a configuration of aphotoelectric conversion element according to a first embodiment of thepresent disclosure.

FIG. 2 is a schematic diagram illustrating a configuration of aphotoelectric conversion element according to a second embodiment of thepresent disclosure.

FIG. 3 is a schematic diagram illustrating a configuration of an imagingelement according to a third embodiment of the present disclosure.

FIG. 4A illustrates an X-ray diffraction spectrum of a film includingcrystalline ZnO, and FIG. 4B illustrates X-ray diffraction spectra ofbuffer layers of Sample Nos. 1 and 2.

FIG. 5 is a diagram illustrating a relationship between oxygenconcentrations and optical transparency.

FIG. 6 is a diagram illustrating energy levels of buffer layers in anexample and comparative examples.

FIG. 7 is a diagram illustrating current characteristics of an exampleand a comparative example.

DESCRIPTION OF EMBODIMENT(S)

Hereinafter, embodiments for carrying out the present disclosure will bedescribed in detail with reference to the accompanying drawings. It isnoted that the present disclosure is not limited to the embodimentsindicated blow. Also, description will be provided in the followingorder.

1. First Embodiment

(Example of Photoelectric Conversion Element Including Inorganic BufferLayer)

2. Second Embodiment

(Example of Photoelectric Conversion Element Including Buffer LayerHaving Laminated Structure)

3. Third Embodiment

(Example of Imaging Device Including Photoelectric Conversion ElementContaining Inorganic Buffer Layer)

<1. First Embodiment>

Firstly, a photoelectric conversion element according to a firstembodiment of the present disclosure will be described. FIG. 1 is across-sectional diagram schematically illustrating a configuration ofthe photoelectric conversion element according to the first embodimentof the present disclosure. As illustrated in FIG. 1, a photoelectricconversion element 10 includes an organic photoelectric conversion layer3 and a buffer layer 4 between a pair of electrodes 1 and 2.

[Electrodes 1 and 2]

The electrodes 1 and 2 can be formed with a transparent material havingconductivity, such as indium-tin oxides (including ITO, Sn-doped In₂O₃,crystalline ITO and amorphous ITO), IFO (F-doped In₂O₃), tin oxide(SnO₂), ATO (Sb-doped SnO₂), FTO (F-doped SnO₂), zinc oxide (includingAl-doped ZnO, B-doped ZnO, and Ga-doped ZnO), indium oxide-zinc oxide(IZO), titanium oxide (TiO₂), spinel-shaped oxides, and oxides having aYbFe₂O₄ structure. Here, the “transparent material” indicates a materialthat does not excessively absorb light incident on the organicphotoelectric conversion layer 3. This also applies to the followingdescription.

Also, of the electrodes 1 and 2, the electrode on which light is notincident may be low in transparency. In this case, the electrode canalso be formed with a metal material such as platinum (Pt), gold (Au),palladium (Pd), chromium (Cr), nickel (Ni), aluminum (Al), silver (Ag),tantalum (Ta), tungsten (W), copper (Cu), titanium (Ti), indium (In),tin (Sn), iron (Fe), cobalt (Co) and molybdenum (Mo), and an alloymaterial containing these metal elements.

It is noted that the electrodes 1 and 2 may be formed with a conductivematerial such as conductive particles containing thepreviously-described metal and metal alloy, impurities-containingpolysilicon, carbon-based materials, oxide semiconductors, carbonnanotubes and graphene. In that case, the electrode may be formed bymixing these conductive materials to a binder resin and curing theresultant paste and ink. Also, the electrodes 1 and 2 can be formed witha conductive polymer material such as poly(3,4-ethylenedioxythiophen)and polystyrene sulfonate. Furthermore, the electrodes 1 and 2 can alsohave a structure in which two or more layers formed with differentmaterials are superimposed on each other.

[Organic Photoelectric Conversion Layer 3]

The organic photoelectric conversion layer 3 can be formed with one ortwo or more organic semiconductor materials. The organic semiconductormaterials to be used here may be any material that can convert lightenergy into electric energy, and is particularly desirably a p-typeorganic semiconductor material. Here, among various organicsemiconductor materials, examples of a material that is reactive togreen color (approximately 490 to 580 nm) may include Pigment Violet 1,3, 4, 5, 5:1, 19 (quinacridone), 23, 27, 29, 31, 32, 33, 34, 35, 36, 37,38, 40, 42, 43, 44, and 50, and Pigment Red 1, 2, 4, 5, 6, 7, 8, 9, 12,13, 17, 21, 22, 23, 24, 31, 32, 38, 48, 49, 50, 51, 52, 53, 54, 64, 68,88, 112, 113, 114, 122, 146, 147, 148, 149, 150, 151, 168, 170, 171,173, 174, 175, 176, 177, 178, 179, 181, 184, 185, 190, 195, 200, 202,206, 207, 208, 209, 214, 216, 221, 224, 225, 242, 251, 254, 255, 259,264, 266, 268, and 269.

Also, examples of a material that is reactive to blue color(approximately 400 to 490 nm) may include naphthalene derivatives,anthracene derivatives, naphthacene derivatives, styrylamine derivativesand bis(azinyl)methene boron complexes. Furthermore, examples of amaterial that is reactive to red color (approximately 580 to 700 nm) mayinclude Nile red, pyran derivatives such as DCM1{4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)4H-pyran} andDCJT {4-(dicyanomethylene)-2-tert-butyl-6-(julolidylstryl)pyran},squarylium derivatives, porphyrin derivatives, chlorin derivatives, andeurodiline derivatives.

It is noted that the organic photoelectric conversion layer 3 can beformed with a combination of two or more organic semiconductormaterials. In that case, the organic photoelectric conversion layer 3may have a bulk hetero structure in which a p-type organic semiconductormaterial and an n-type organic semiconductor material are mixed.

[Buffer Layer 4]

The buffer layer 4 is disposed between the organic photoelectricconversion layer 3 and the electrode 2 on which light is incident, andforms an energy barrier. The buffer layer 4 of the photoelectricconversion element 10 according to the present embodiment is formed withan amorphous inorganic material, and has an energy level of 7.7 to 8.0eV and a difference in a HOMO energy level from the organicphotoelectric conversion layer of 2 eV or more.

When the inorganic material forming the buffer layer 4 is amorphous,stress inside the film is reduced, and an intermediate level isprevented from being formed during the lamination of the device. Thus,dark currents can be readily suppressed. The organic photoelectricconversion layer 3 can be prevented from being damaged during formationof the buffer layer 4. Also, when the energy level is 7.7 to 8.0 eV, andthe difference in an HOMO energy level from the organic photoelectricconversion layer is 2 eV or more, an energy barrier becomes higher thanin the past. Thus, dark currents can be suppressed.

An example of the inorganic material forming such a buffer layer 4 mayinclude metal oxides, and particularly preferably includes a metal oxidesemiconductor. Specifically, the buffer layer 4 is preferably formedwith two or more metal oxides. It is preferable that at least one of themetal oxides be a metal oxide semiconductor. Accordingly, there can beachieved a buffer layer that is excellent in transparency and has a highenergy barrier.

Here, examples of metal oxides used in the buffer layer 4 may includezinc oxide, silicon oxide, tin oxide, niobium oxide, titanium oxide,molybdenum oxide, aluminum oxide, In—Ga—Zn-based oxides (IGZO),magnesium oxide and hafnium oxide, and preferably a combination thereof.Particularly, from the viewpoint of transparency, a high energy barrierand easiness of formation, the buffer layer 4 is preferably formed withzinc oxide and aluminum oxide.

Also, it is preferable that the buffer layer 4 be formed with atransparent material, and have a relative refractive index to theelectrode 2 of 0.3 or less. When the relative refractive index to theelectrode 2 is 0.3 or less, scattering of incident light is suppressed.Thus, incidence efficiency on the organic photoelectric conversion layer3 can be increased.

On the other hand, the thickness of the buffer layer 4 is, but notparticularly limited to, preferably 3 to 300 nm, from the viewpoint ofthe incident light amount on the organic photoelectric conversion layer3 and the setting of a film thickness in terms of an optical design inthe laminated structure with the organic photoelectric conversion layer3. Also, the surface resistance of the buffer layer 4 is preferably 100kΩ/□ or more, from the viewpoint of the securement of insulation.

[Substrate]

The previously-described electrodes 1 and 2, organic photoelectricconversion layer 3 and buffer layer 4 can be formed, for example, on asubstrate. The substrate (not illustrated) may be any substrate that cansupport these layers, and the material properties and shape of thesubstrate are not particularly limited. Examples of a material thatconstitutes the substrate may include synthetic resins such aspolymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyvinylphenol (PVP), polyether sulfone (PES), polyimide, polycarbonate (PC),polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).

When the substrate is formed with synthetic resins, the form thereof maybe a film, a sheet, and the like, as well as a plate. Moreover, the useof a substrate having flexibility enables, for example, an electronicdevice to be incorporated or integrated into an electronic apparatushaving a curved surface. Also, the substrate may be formed with aninorganic material such as mica, glass and quartz. Furthermore, as thesubstrate, there can be used, for example, a product obtained by formingan insulating film including silicon oxide, silicon oxynitride, aluminumoxide, metal oxides, metal salts, or the like, on the surface of variousglass substrates, quartz substrates, silicon substrates, metalsubstrates, carbon substrates, or the like.

Moreover, for example, when light is received on the substrate side, thesubstrate is preferably formed with a transparent material. it is notedthat although the surface of the substrate is preferably smooth, it maybe uneven to a degree that does not influence the characteristics of theorganic photoelectric conversion layer 3. Also, the surface of thesubstrate may be subjected to a surface treatment for improving adhesionwith an electrode formed on the substrate.

[Other Layers]

The photoelectric conversion element 10 according to the presentembodiment may include a buffer layer, shock-absorbing layer and thelike including an organic material, in addition to thepreviously-described electrodes 1 and 2, organic photoelectricconversion layer 3 and buffer layer 4,

The organic buffer layer is disposed on the upper or lower surface ofthe buffer layer 4, and can be formed by a method of deposition,spraying, coating, printing, or the like. The use of a combination ofthis organic buffer layer and the previously-described buffer layer 4enables the effects by a buffer layer of promoted carrier injection anda hole injection barrier to be further improved.

The shock-absorbing layer is disposed between the electrode 2 and thebuffer layer 4, and disposed for relaxing stresses generated in thelower layers such as the electrode 1, the organic photoelectricconversion layer 3, the buffer layer 4 and the like during the formationof the electrode 2. The shock-absorbing layer can be formed with, forexample, an organic material, by a method of deposition, spraying,coating, printing or the like. The disposition of this shock-absorbinglayer enables durability and reliability of the photoelectric conversionelement 10 to be improved.

[Manufacturing Method]

When manufacturing the photoelectric conversion element 10 according tothe present embodiment, a step of forming the lower electrode 1, a stepof forming the organic photoelectric conversion layer 3, a step offorming the buffer layer 4, and a step of forming the upper electrode 2are performed in this order.

(Step of Forming Lower Electrode 1)

The forming method of the electrode 1 is not particularly limited, andcan be appropriately selected depending on an electrode material.Specifically, examples of the forming method of the electrode 1 mayinclude a physical vapor deposition method (PVD method) such as a vacuumvapor deposition method, a reactive vapor deposition method, varioussputtering methods, an electron beam vapor deposition method and an ionplating method, various chemical vapor deposition methods (CVD methods)including a pyrosol method, a method of thermally decomposing anorganometallic compound, a spray method, a dip method and an MOCVDmethod, and various plating methods such as an electroless platingmethod and an electrolytic plating method, a lift-off method, a sol-gelmethod, an electrodeposition method, and a shadow mask method. Acombination of these may be performed. Also, these techniques and apatterning technique may be combined.

(Step of Forming Organic Photoelectric Conversion Layer 3)

The forming method of the organic photoelectric conversion layer 3 isnot particularly limited. Various CVD methods including a coatingmethod, a PVD method, and an MOCVD method can be applied. Here, examplesof the coating method may include a spin coating method, a dippingmethod, a cast method, various printing methods such as a screenprinting method, an ink-jet printing method, an offset printing methodand a gravure printing method, and various coating methods such as astamp method, a spray method, an air doctor coater method, a bladecoater method, a rod coater method, a knife coater method, a squeezecoater method, a reverse roll coater method, a transfer roll coatermethod, a gravure coater method, a kiss coater method, a cast coatermethod, a spray coater method, a slit orifice coater method and acalendar coater method. At that time, as a solvent, there can be usednon-polar or low polar organic solvents such as toluene, chloroform,hexane, and ethanol.

Also, examples of the PVD method may include a vacuum vapor depositionmethod using various heating methods such as an electron beam heatingmethod, a resistance hearing method, a lamp heating method and ahigh-frequency induction heating method, a plasma vapor depositionmethod, various sputtering methods such as a bipolar sputtering method,a direct current sputtering method, a direct current magnetronsputtering method, a high-frequency sputtering method, a magnetronsputtering method, an ion beam sputtering method and a bias sputteringmethod, a DC (direct current) method, an RF method, a multi-cathodemethod, an activated reactive method, an electric field vapor depositionmethod, and various ion plating methods such as a high-frequency ionplating method and a reactive ion plating method. Furthermore, whenintegrating the light-receiving element according to the presentembodiment, a method of forming a pattern on the basis of a pulsed laserdeposition (PLD) method can be adopted.

(Step of Forming Buffer Layer 4)

The buffer layer 4 can be formed with the previously-described materialsby, for example, a sputtering method. At that time, a film may be formedwith a target which contains oxygen. However, a film is desirably formedwhile introducing oxygen in order to stably ensure the transparency ofthe buffer layer 4. At that time, the amount of oxygen to be introducedis, but not particularly limited to, preferably 2% or less. Even whenthe oxygen concentration is excessively increased, the obtainedtransmittance of the buffer layer does not decrease. Rather, the organicphotoelectric conversion element is damaged in some cases.

(Step of Forming Upper Electrode 2)

The upper electrode 2. can be formed in a similar manner to thepreviously-described lower electrode 1. Therefore, description about aspecific method thereof will be omitted.

The conventional organic photoelectric conversion element usuallyincludes a buffer layer formed with an organic material. For thisreason, an injection barrier cannot be sufficiently formed, causing darkcurrents to increase. On the contrary, the photoelectric conversionelement according to the present embodiment includes a buffer layerformed with an amorphous inorganic material. Therefore, there can beachieved a buffer layer having excellent transparency and high potentialenergy without damaging the organic photoelectric conversion layer. As aresult, a high energy barrier can be formed. This enhances thesuppression effect of dark currents and improves pressure resistance.

The photoelectric conversion element according to the present embodimentcan achieve various optical sensor elements such as a highly sensitiveimaging element and an infrared sensor. The photoelectric conversionelement according to the present embodiment is particularly suitable asan imaging element.

<2. Second Embodiment>

Next, a photoelectric conversion element according to a secondembodiment of the present disclosure will be described. FIG. 2 is across-sectional diagram schematically illustrating a configuration ofthe photoelectric conversion element according to the second embodimentof the present disclosure. It is noted that in FIG. 2, a constituentwhich is the same as that of the photoelectric conversion element 10 inFIG. 1 is assigned with the same reference numeral, and descriptionthereof will he omitted. As illustrated in FIG. 2, a photoelectricconversion element 20 according to the present embodiment is similar tothe previously-described photoelectric conversion element 10 accordingto the first embodiment, except that a buffer layer 14 is constituted bya plurality of layers each having a different energy level.

[Buffer Layer 14]

Each of inorganic buffer layers 14 a and 14 b constituting the bufferlayer 14 contains one or two or more inorganic materials, and has adifferent energy level from each other. Accordingly, a high energyharrier is formed so that a hole injection barrier can be established,in a similar manner to the previously-described photoelectric conversionelement according to the first embodiment. Thus, dark currents can besuppressed. It is noted that as an inorganic material to be contained ineach of the inorganic buffer layers 14 a and 14 b, materials similar tothose of the previously-described buffer layer 4 of the photoelectricconversion element 10 according to the first embodiment can be used.

Also, in the case of the laminated structure, the buffer layer 14preferably has a thickness of 3 to 300 nm and a surface resistance of100 kΩ/□ or more, in a similar manner. As described herein, thethickness and surface resistance are not the thickness of each layer,but the value for the whole laminated buffer layer.

The photoelectric conversion element 20 according to the presentembodiment includes the buffer layer 14 which is constituted by two ormore inorganic buffer layers 14 a and 14 b each having a differentenergy level. Accordingly, an injection barrier is formed in a reliableand step-wise manner compared to in the case of a one-layer structure.Thus, hole injection can be effectively suppressed. Also, in thephotoelectric conversion element 20 according to the present embodiment,carrier injection is performed in a step-wise and smooth manner.Therefore, deactivation of electron injection can also be suppressed. Itis noted that the configuration and effect other than the above in thephotoelectric conversion element 20 according to the present embodimentare similar to those in the previously-described first embodiment. Also,the photoelectric conversion element according to the present embodimentis suitable as various optical sensors such as an imaging element and aninfrared sensor.

<3. Third Embodiment>

[Configuration]

Next, an imaging device according to a third embodiment of the presentdisclosure will be described. The imaging device according to thepresent embodiment includes as an imaging element thepreviously-described photoelectric conversion element 10 or 20 accordingto the first or second embodiment. FIG. 3 is a diagram schematicallyillustrating a configuration of the imaging device according to thepresent embodiment. It is noted that although the photoelectricconversion element 10 according to the first embodiment is used in FIG.3, an imaging device 30 according to the present embodiment may includethe photoelectric conversion element 20 according to the secondembodiment in place of the photoelectric conversion element 10.

As illustrated in FIG. 3, the imaging device 30 according to the presentembodiment includes a plurality of photoelectric conversion elements 10in a matrix shape, for example, on a semiconductor substrate such as anSi substrate. The region in which these photoelectric conversionelements 10 are arranged functions as an imaging region 31. it is notedthat when integrating the previously-described photoelectric conversionelement 10 or 20 according to the first or second embodiment, a methodof forming a pattern on the basis of a pulsed laser deposition (PLD)method or the like can be adopted,

Also, the imaging device 30 according to the present embodimentincludes, as peripheral circuits of the imaging region 31, a verticaldrive circuit 32, a column signal processing circuit 33, a horizontaldrive circuit 34, an output circuit 35, a control circuit 36, and thelike.

The control circuit 36 generates a clock signal and a control signalwhich serve as criteria for actions by the vertical drive circuit 32,the column signal processing circuit 33, and the horizontal drivecircuit 34, on the basis of a vertical synchronization signal, ahorizontal synchronization signal, and a master clock. The clock signaland control signal generated in this control circuit 36 are inputted tothe vertical drive circuit 32, the column signal processing circuit 33and the horizontal drive circuit 34.

The vertical drive circuit 32 is constituted by, for example, a shiftregister, and selectively scans each of the photoelectric conversionelements 10 in the imaging region 31 row by row sequentially in avertical direction. A pixel signal based on a current (signal) generatedin the vertical drive circuit 32 according to the received light amountin each of the photoelectric conversion elements 10 is transmitted tothe column signal processing circuit 33 via a vertical signal line 37.

The column signal processing circuit 33 is disposed, for example, foreach column of the photoelectric conversion elements 10, and performssignal processing of noise removal and signal amplification to signalsoutputted from a row of the photoelectric conversion elements 10 foreach photoelectric conversion element with signals from black referencepixels (not illustrated, formed around an effective pixel region). Also,in an output stage of the column signal processing circuit 33, ahorizontal selection switch (not illustrated) is connected between thecolumn signal processing circuit 33 and a horizontal signal line 38.

The horizontal drive circuit 34 is constituted by, for example, a shiftresistor. Then, in this horizontal drive circuit 34, a horizontalscanning pulse is sequentially outputted thereby to sequentially selecteach of the column signal processing circuits 33, and a signal isoutputted from each of the column signal processing circuit 33 to thehorizontal signal line 38.

The output circuit 35 performs signal processing to the signalsequentially supplied from each of the column signal processing circuits33 via the horizontal signal line 38, and outputs the resultant signal.

These circuits can be constituted by known circuits. Also, the circuitconfiguration in the imaging device 30 according to the presentembodiment is not limited to the previously-described configuration, andother circuit configurations such as various circuits which are used in,for example, a conventional CCD imaging device and a CMOS imaging devicemay be used.

The imaging device according to the present embodiment includes thefirst and second photoelectric conversion elements in which darkcurrents are suppressed. Therefore, there can be achieved an organicphotoelectric conversion device having higher sensitivity and pressureresistance than conventional devices. It is noted that thepreviously-described first and second photoelectric conversion elements10 and 20 can also be used, other than in the previously-describedimaging device 30, in various optical sensors such as an infraredsensor.

Additionally, the present technology may also be configured as below

(1)

A photoelectric conversion element, including:

a first electrode;

an organic photoelectric conversion layer disposed in a layer upper thanthe first electrode, the organic photoelectric conversion layerincluding one or two or more organic semiconductor materials;

a buffer layer disposed in a layer upper than the organic photoelectricconversion layer, the buffer layer including an amorphous inorganicmaterial and having an energy level of 7.7 to 8.0 eV and a difference ina HOMO energy level from the organic photoelectric conversion layer of 2eV or more; and

a second electrode disposed in a layer upper than the buffer layer.

(2)

The photoelectric conversion element according to (therein the bufferlayer is formed with two or more metal oxides.

(3)

The photoelectric conversion element according to (2), wherein at leastone of the metal oxides is a metal oxide semiconductor.

(4)

The photoelectric conversion element according to any one of (1) to (3),wherein the buffer layer is formed with two or more metal oxidesselected from the group consisting of zinc oxide, silicon oxide, tinoxide, niobium oxide, titanium oxide, molybdenum oxide, aluminum oxide,in-Ga—Zn-based oxides (IGZO), magnesium oxide and hafnium oxide.

(5)

The photoelectric conversion element according to any one of (1) to (4),wherein the buffer layer is constituted by a plurality of layers eachhaving a different energy level.

(6)

The photoelectric conversion element according to any one of (1) to (5),wherein the buffer layer has a thickness of 3 to 300 nm.

(7)

The photoelectric conversion element according to any one of (1) to (6),wherein the buffer layer has a surface resistance of 100 kΩ/□ or more.

(8)

The photoelectric conversion element according to any one of (1) to (7),wherein the second electrode is formed with a transparent material.

(9)

The photoelectric conversion element according to (8), wherein thebuffer layer is formed with a transparent material, and has a relativerefractive index to the second electrode of 0.3 or less.

(10)

The photoelectric conversion element according to any one of (1) to (9),wherein the buffer layer is formed by a sputtering method.

(11)

An imaging device including the photoelectric conversion elementaccording to any one of (1) to (10).

(12)

An optical sensor including the photoelectric conversion elementaccording to any one of (1) to (10).

(13)

The optical sensor according to (12), which is an infrared sensor.

(14)

A method of manufacturing a photoelectric conversion element, the methodincluding:

a step of forming a first electrode;

a step of forming, in a layer upper than the first electrode, an organicphotoelectric conversion layer including one or two or more organicsemiconductor materials;

a step of forming, in a layer upper than the organic photoelectricconversion layer, a buffer layer including an amorphous inorganicmaterial and having an energy level of 7.7 to 8.0 eV and a difference ina HOMO energy level from the organic photoelectric conversion layer of 2eV or more; and

a step of forming, in a layer upper than the buffer layer, a secondelectrode.

(15)

The method of manufacturing the photoelectric conversion elementaccording to (14), including forming the buffer layer by a sputteringmethod.

(16)

The method of manufacturing the photoelectric conversion elementaccording to (15), including forming a film of the buffer layer whileintroducing Oxygen.

It is noted that the effects described herein are merely exemplary andnot limiting, and other effects may exist.

EXAMPLES

Hereinafter, the effects of the present invention will be specificallydescribed with reference to examples of the present invention. In thepresent examples, zinc oxide (ZnO) as a main component (content: 50% bymass or more) and Al₂O₃, SiO₂, MgO and SnO₂ as an auxiliary componentwere used to form buffer layers of Sample Nos. 1 to 4 indicated in Table1 below. The evaluation results of the buffer layers are indicated inTable 2 below

TABLE 1 Auxiliary Sample No. Main component component 1 ZnO Al₂O₃ SiO₂ 2ZnO Al₂O₃ MgO 3 ZnO Al₂O₃ SnO₂ 4 ZnO MgO SiO₂

TABLE 2 No. 1 No. 2 No. 3 No. 4 I.P. (eV) 7.8 7.8 7.7 7.7 Eg. (eV) 3.13.0 2.9 3.0 Conduction Band (eV) 4.7 4.8 4.8 4.7 ΔE 2.1 2.1 2.0 2.0

It is noted that ionization potential (I.P.) indicated in Table 2 abovewas obtained by ultraviolet photoelectron spectroscopy (UPS). Also, bandgap (Eg) was calculated from an absorption edge. Furthermore, ΔE is adifference between the HOMO value of a photoelectric conversion layerand the ionization potential (I.P.) (ΔE=photoelectric conversion layerHOMO−I.P.) of each buffer layer.

FIG. 4A illustrates an X-ray diffraction spectrum of a film includingcrystalline ZnO, and FIG. 4B illustrates X-ray diffraction spectra ofthe buffer layers of Sample Nos. 1 and 2. A peak indicatingcrystallinity is observed in the spectrum of ZnO indicated in FIG. 4A.However, a peak indicating crystallinity is not observed for the bufferlayers of Sample Nos. 1 and 2. This demonstrated that the buffer layersare amorphous.

Also, a film was formed with changed oxygen concentrations. The obtainedbuffer layer (film thickness: 100 nm) was checked for opticaltransparency. FIG. 5 is a diagram illustrating a relationship betweenthe oxygen concentrations during film formation and the transmittanceproperties of a buffer layer. As illustrated in FIG. 5, a buffer layerformed with introduced oxygen had higher optical transmittance than abuffer layer formed without introduced oxygen.

FIG. 6 is a diagram for comparison among the energy level of the bufferlayer of the previously-described example, the energy levels ofconventional buffer layers including an organic material, and the energylevel of an organic photoelectric conversion layer. As illustrated inFIG. 6, the buffer layer of the example (Sample No. 1) had a higherenergy level than conventional buffer layers including an organicmaterial, and the difference (ΔE) from the HOMO value of thephotoelectric conversion layer was also 2 eV or more.

Also, the evaluation result of dark currents pressure resistanceperformance is indicated in Table 3 below. Furthermore, currentproperties of the buffer layers of an example and a comparative exampleare illustrated in FIG. 7.

TABLE 3 Jdk (−1 V) Jdk (−3 V) Δ Jdk Example 1 1.2 × 10⁻¹⁰ 2.0 × 10⁻¹⁰0.8 × 10⁻¹⁰ Example 2 1.3 × 10⁻¹⁰ 1.9 × 10⁻¹⁰ 0.6 × 10⁻¹⁰ ComparativeExample 1 1.7 × 10⁻¹⁰ 3.4 × 10⁻¹⁰ 1.7 × 10⁻¹⁰

The above results demonstrated that the buffer layers of Examples I and2 are amorphous, and have a high energy level and improved pressureresistance properties. That is, it was confirmed that according to thepresent invention, a high energy barrier can be formed by the bufferlayer to improve the suppression effect of dark currents.

REFERENCE SIGNS LIST

1, 2 electrode

3 organic photoelectric conversion layer

4, 14 buffer layer

10, 20 photoelectric conversion element

30 imaging device

31 imaging region

32 vertical drive circuit

33 column signal processing circuit

34 horizontal drive circuit

35 output circuit

36 control circuit

37 vertical signal line

38 horizontal signal line

What is claimed is:
 1. A photoelectric conversion element, comprising: afirst electrode formed of a first transparent material; a secondelectrode formed of a second transparent material; an organicphotoelectric conversion layer disposed between the first electrode andthe second electrode, the organic photoelectric conversion layerincluding one or more organic semiconductor materials; and a bufferlayer disposed between the first electrode and the organic photoelectricconversion layer, the buffer layer including an amorphous inorganicmaterial and having an energy level of 7.7 to 8.0 eV, wherein adifference in a highest occupied molecular orbital (HOMO) energy levelfrom the organic photoelectric conversion layer and the buffer layer is2 eV or more, and wherein the buffer layer is formed with a thirdtransparent material having a refractive index with respect to at leastthe first electrode or the second electrode of 0.3 or less.
 2. Thephotoelectric conversion element according to claim 1, wherein thebuffer layer is formed with two or more metal oxides.
 3. Thephotoelectric conversion element according to claim 2, wherein at leastone of the metal oxides is a metal oxide semiconductor.
 4. Thephotoelectric conversion element according to claim 2, wherein thebuffer layer is formed with two or more metal oxides selected from thegroup consisting of zinc oxide, silicon oxide, tin oxide, niobium oxide,titanium oxide, molybdenum oxide, aluminum oxide, In—Ga—Zn-based oxides(IGZO), magnesium oxide and hafnium oxide.
 5. The photoelectricconversion element according to claim 1, wherein the buffer layerincludes a plurality of layers each having a different energy level. 6.The photoelectric conversion element according to claim 1, wherein thebuffer layer has a thickness of 3 to 300 nm.
 7. The photoelectricconversion element according to claim 1, wherein the buffer layer has asurface resistance of 100 kΩ/□ or more.
 8. The photoelectric conversionelement according to claim 1, wherein the first transparent material andthe second transparent material is same.
 9. The photoelectric conversionelement according to claim 1, wherein the buffer layer is formed by asputtering method.
 10. An imaging device comprising the photoelectricconversion element according to claim
 1. 11. An optical sensorcomprising the photoelectric conversion element according to claim 1.12. The optical sensor according to claim 11, which is an infraredsensor.